Lawyers for Michael T. Flynn, U.S. President Donald Trump's former national security adviser, notified the president's legal team in recent days that they could no longer discuss the special counsel's investigation, according to four people involved in the case, an indication that Flynn is cooperating with prosecutors or negotiating such a deal, New York Times reported.

Earlier on Thursday, NBC News reported that Special Counsel Robert Mueller's investigation on the Trump campaign, Russia and the 2016 U.S. election has honed in on a former business associate of ex-National Security Adviser Michael Flynn for his role in failing to disclose that Flynn's lobbying firm was conducting work for foreign governments.

That agreement has been terminated, the four people said. Defense lawyers frequently share information during investigations, but they must stop when doing so would pose a conflict of interest. It is unethical for lawyers to work together when one client is cooperating with prosecutors and another is still under investigation, the report said.

The notification alone does not prove that Flynn is cooperating with Mueller. Some lawyers withdraw from information-sharing arrangements as soon as they begin negotiating with prosecutors. And such negotiations sometimes fall apart, according to the NYT.

Still, the notification led Trump’s lawyers to believe that Flynn — who, along with his son, is seen as having significant criminal exposure — has, at the least, begun discussions with Mueller about cooperating, the report underlined.

Flynn is regarded as loyal to Trump, but he has in recent weeks expressed serious concerns to friends that prosecutors will bring charges against his son, Michael Flynn Jr., who served as his father's chief of staff and was a part of several financial deals involving the elder Flynn that Mueller is scrutinizing, NYT said.

Mueller is investigating whether Flynn was involved in an alleged plan to seize Islamic preacher Fethullah Gülen and deliver him to Turkey in exchange for millions of dollars, the Wall Street Journal had reported. Under the plan, Flynn, who was fired by Trump after just 24 days in the job, and his son, Michael Flynn Jr, were to receive up to $15 million for forcibly removing Gülen from his U.S. home and delivering him to the Turkish government, people familiar with the investigation told the Journal.